1. Field of the Invention
The present invention relates to a drive device for a varifocal lens system, and more particularly to a drive device for driving a varifocal lens system comprising a varifocal optical system which includes at least a first group of lenses and a second group of lenses both disposed on the same optical axis and having changeable focal length, the first group of lenses being used as a focusing lens group.
2. Description of the Prior Art
In these days, with progress in advanced electronics and electrically operated mechanisms for use in cameras, a variety of so-called varifocal cameras have been proposed which can set a plurality of focal lengths of a photo-taking optical system or modify the focal length thereof infinitely by a motor or the like energized upon one-touch operation. Hitherto, this type varifocal camera has generally been so designed that a cam ring having a cam groove defined therein is rotated to drive (move) a lens frame having a cam pin slidably fitted in the cam groove along the optical axis, to thereby vary the focal length.
While such a conventional device is advantageous in that the construction is relatively simple, it has suffered from a problem that the cam ring requires a high degree of machining accuracy, which pushes up the manufacturing cost, and the length of the cam ring along the optical axis cannot be reduced, which eventually prevents compact design of a mirror cylinder.
As disclosed in Japanese Patent Laying-Open No. 60-79314 (1985), it has also been proposed to provide a single drive screw in place of the cam ring, for driving a lens frame along the optical axis. This type drive device has, however, accompanied problems that a lens is hard to smoothly move because driving forces are concentrated on a portion of the lens frame, and that the lens frame is inclined and hence the lens plane, which has to remain vertical with respect to the optical axis, cannot hold its vertical posture, whereby the intended optical capability cannot be effected.
As an alternative, there is also known a device of the type that a lens frame, a lens barrel or the like is housed (fully retracted) into a stowed position by means of a cam ring for compact design.
In such a case, however, a cam member for driving the lens frame or the like into a stowed position must be provided in addition to a cam member for driving the lens frame or the like to change the focal length. This arises a problem of increasing a pressure angle of the cam. To solve this problem, the diameter of the cam ring must be enlarged. Eventually, the lens barrel is increased in its diameter, thereby making it difficult to achieve compact size on a whole.
Further, in order to solve the above problem, the inventors have previously invented a device comprising a lens barrel divided into a plurality of frames, in which a lens barrel frame is disposed inside a fixed frame, a first group frame for supporting a first group of lenses is disposed inside the lens barrel frame, and a second group frame for supporting a second group of lenses is disposed inside the first group frame, in such a manner that these lens barrel frames, first group frame and second group frame are all movable along the optical axis to separately advance and retract. In the course of developing this device, the inventors have faced a problem as to what is suitable means for driving the second group frame. As the simplest expedient, for example, it is conceivable to directly drive the second group frame through a speed reducing mechanism by a motor or the like, as a driving source, disposed on the fixed frame. But, since the amount of movement of the second group frame with respect to the fixed frame is fairly large, the drive mechanism becomes necessarily large in size and complicated, and adjustment of the gear ratios is troublesome. When driving forces are transmitted from the fixed frame to the second group frame disposed inside the lens barrel frame, another problem arises in that a cut-out must be bored in the lens barrel frame in size corresponding to the sum of amounts of relative movements of both the first group frame and the lens barrel frame, and hence dealing with changes in the amount of movement of the lens barrel frame becomes difficult.
As disclosed in Japanese Patent Laid-Open No. 60-79314 (1985), it has also been proposed to provide a single drive screw in place of the cam ring, for driving a lens frame along the optical axis This type drive device has, however, accompanied problems that a lens is hard to smoothly move because driving forces are concentrated on a portion of the lens frame, and that the lens frame is inclined and hence the lens plane, which has to remain vertical with respect to the optical axis, cannot hold its vertical posture, whereby the intended optical capability cannot be effected. In other words, the lens will lose its stable posture as it moves.
Moreover, in order to solve the problem of holding a vertical posture of the lens, it is further conceivable to use a plurality of driver screws. But this solution brings forth another new problem that a transmission mechanism for transmitting driving forces to the respective driver screws is complicated, or that the lens barrel frame and so on are increased in the diameter significantly. Stated otherwise, it is very difficult to incorporate the transmission mechanism under requirements of reducing the diameter of the lens barrel frame, without interrupting the effective light flux.